Move build/howl root anywhere

[eelstork]

Currently:

• howl root under Assets/
• build root under Assets/~build (may be moved/renamed safely, anywhere in the Assets directory, provided Unity is closed while doing so)

Pro - putting derived products in a ~build dir is intuitive; mixing Howl and C# in the same project remains intuitive.
Con - imported C# sources are moved. In general not a problem, yet some packages/tools may not like this. Also, until own UI for asmdef is available, managing that feels clunky.

Initially, was doing it the other way (C# under Assets/, Howl under Assets/[ProjectName].Howl/). Not intuitive; however, does not require moving files.

Anywhere would imply side by side, one inside another or perhaps outside Unity projects, or in packages. Likely dev/deployment scenarios for any of these.